(a) Field of the Invention
The present invention relates to an electrode material, a method for its preparation, an electrode, and a battery comprising the electrode, and more particularly, to an electrode material having a strong binding strength among active materials or between an active material and a collector, having flexibility, a higher charge-discharge capacity, and enhanced cycle life characteristics, a method for its preparation, an electrode, and a battery comprising the same.
(b) Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments.
Because the capacity of a battery is proportional to the amount of the active material, it is important to fill as much active material as possible in the electrode plate by eliminating materials other than active materials, in order to obtain a lithium secondary battery having a high energy density as well as to increase a capacity per unit weight of the active material.
Polyvinylidene fluoride (PVdF), which is currently commonly used as a negative electrode binder, is soluble in an organic solvent such as N-methyl-2-pyrrolidone. Although PVdF is not specially used as a binder, it is found that it provides a binding strength to an electrode by adding 8 to 10 wt % based on the carbon amount, and in addition, it is compatible with graphite materials.
However, PVdF can coat the active materials even in the state that fibers are fully bound, so that capacity, efficiency, and other inherent battery properties of an active material are not optimized. For the complete intercalation/deintercalation of lithium ions into an active material, electrode impedance should be drastically reduced, but a common binder is non-conductive. Therefore, it is necessary to reduce the amount of binder added as well as to increase the conductivity of the binder. It is suggested to incorporate a conductive polymer into a binder to obtain excellent battery properties, which is not accomplished with the conventional methods.
The PVdF has strong binding strength, but low flexibility. The low flexibility of PVdF can easily deteriorate cycle life characteristics of a lithium secondary battery due to breaking of the bond between active materials when the active material is a carbon material such as a natural graphite having a small surface pitch and a consequently high ratio of expansion to contraction during charging and discharging. Therefore, in order to absorb the expansion and contraction stresses of the active material during charging and discharging, it has been suggested to adopt a binder having elasticity.
With respect to safety, a binder such as PVdF, which is soluble in an organic solvent, is harmful to humans and the environment, and the organic solvent should be recovered. Therefore, it is advantageous to use an aqueous binder that is environmentally friendly and does not need to be recovered.
An exemplary aqueous binder for a lithium secondary battery is latex such as styrene-butadiene rubber (SBR). SBR has a high elasticity, and it is expected to help an electrode relieve expansion and contraction during charging and discharging, when the SBR is used for a binder with a thickener such as cellulose. However, since the latex binder is adhesive, it has a smaller surface area in contact with an active material compared to polyvinylidene fluoride. Therefore, the active material may be easily separated from the electrode due to weakening of binding strength between active materials, and the cycle life characteristics of a battery comprising the electrode may deteriorate compared to those of a battery comprising the PVdF binder.
In particular, artificial graphite has a small specific surface area and bad wettability. Therefore, when a binder with only latex and a thickener is applied to the artificial graphite, the active material can easily separate from the electrode during hundreds of repeated charge and discharge cycles.